1. Field of the Invention
The invention concerns an RFID and NEC antenna circuit.
RFID is the abbreviation for Radio Frequency Identification.
NFC is the abbreviation for Near Field Communication.
This is a technique which allows identification of objects using a memory chip or an electronic device which, by means of a radio antenna, is capable of transmitting information to a specialized reader.
2. Description of Related Art
RFID/NFC technology is used in numerous areas, for example in mobile telephones, personal digital assistants PDAs, computers, contactless card readers, the cards themselves which are to be read without contact, but also passports, identification or description tags, USB keys, SIM and (U)SIM cards called “RFID or NEC SIM card”, stickers for Dual or Dual Interface cards (the sticker itself having an RFID/NFC antenna), watches.
In RFID/NFC technology, the antenna of a first RFID circuit (Reader) electromagnetically radiates a radiofrequency signal over a certain distance which contains data that is to be received by the antenna of a second REID circuit (transponder) which may optionally reply by data by charge modulation to the first circuit. Each REID circuit has its antenna operating at its natural resonance frequency.
As a general rule, the problems with RFID antenna circuits relate to the efficiency of the magnetic antenna of the transponder and reader i.e. to the efficiency of coupling by mutual inductance between the two magnetic antennas, to the transmission of energy and information between the electronic part and its antenna, and to the transmission of energy and information between the two antennas of the RFID system.
The chief objective is to gain in radio efficiency (emitted or captured magnetic field power, coupling, mutual inductance, etc.) by the antenna without losing any signal quality (data distortion, antenna bandwidth, etc.) whether emitted or received.
Antennas with reduced surface areas (30×30 mm) are becoming increasingly more seen, even largely reduced surface areas (5×5 mm) for applications such as cards or μCards, stickers, small readers, option or detachable readers in mobile telephony, in USB keys, in SIM cards.
In addition to a reduced (<16 cm2) or largely reduced (<4 cm2) surface area, very often there are very strong mechanical or electric constraints such as the presence of a battery, a screen or display, a conductor support in the field very close to the antenna.
These various electric and mechanical constraints on the surface lead to reduced efficiency of the antenna, to loss of coupling efficiency, to loss of signal power emitted or received by the antenna, and to reduced communication distance or reduced transmission of energy or information.
For antennas of reasonable size (>16 cm2) as for antennas of reduced (<16 cm2) or largely reduced (<4 cm2) surface area, increasingly greater needs are being encountered regarding the need for power on the emitted or captured magnetic field, the bandwidth of the radio channel to meet ever increasing data rates and standards in force such as ISO 14443 (e.g. for transport, identity, etc.), ISO 15693 (e.g. for tags) and RFID/NFC specifications for the banking sector (EMVCO).
Document U.S. Pat. No. 7,212,124 for example describes an information device for mobile telephone, comprising an antenna coil formed on a substrate, a sheet of magnetic material, an integrated circuit and resonance capacitors connected to the antenna coil. The integrated circuit communicates with an outside apparatus through use by the antenna coil of a magnetic field. A depression serving as a battery receiving section is formed on one part of the surface of the case and covered by the battery cover. The battery, antenna coil and sheet of magnetic material are housed in the depression. A film of vacuum evaporated metal or a conductive material coating is applied to the case, while no film of vacuum evaporated metal or coating of conductive material is applied to the battery cover. The antenna coil is arranged between the battery cover and the battery, whilst the sheet of magnetic material is arranged between the antenna coil and the battery in the depression. The antenna coil has an intermediate tap, the resonance capacitors are connected to both ends of the antenna coil, and the integrated circuit is connected in the centre between one of the ends of the antenna coil and the intermediate tap.
This device has numerous disadvantages.
It only functions in mobile telephones. On account of the presence of a battery, the antenna must have a very high quality factor before its integration. However, a quality factor having such a high value is not suitable for RFID/NFC antenna circuits, readers or transponders (cards, tags, USB keys). In a mobile telephone, the reason this high value quality factor exists is that electric and mechanical constraints overwhelm the original quality factor of the antenna. For conventional applications or without these constraints, this quality coefficient of the antenna would be too high and would generate a much reduced antenna bandwidth at −3 dB, hence very severe filtering of the modulated emitted or received HF signal through charge modulation (subcarrier of 13.56 MHz at ±847 kHz, ±424 kHz, ±212 kHz, etc.) and too high emitted or received power. Also the coupling with said antenna, again for conventional applications or without these constraints, would be such that at a short distance between the 2 antennas (<2 cm for example) the mutual inductance created would be such that it would fully mistune the frequency tuning of the two antennas, would cause the power radiated by the reader to collapse, could saturate the radio stages of the silicon chip and even lead to possible destruction of the transponder silicon, this silicon not having infinite calorific dispersion capacity.
Therefore document US-A1-2008/0150693 for example describes an antenna device essentially for reader mode operation. It has a conventional arrangement of a series inductance, an arrangement of two parallel inductances and finally an arrangement of two series inductances with a third inductance parallel to one of the two series inductances. The embodiments proposed notably require two different surfaces, one large and one small, either on the same inductance or on two inductances. The objective of the two latter embodiments is to allow amplification of the signal emitted in the centre of the antenna by a small parallel inductance, and in a third embodiment, to eliminate radiation holes over a location lying between the arrangement of the two antenna surfaces.
One of the disadvantages of the antenna device according to document US-A1-2008/0150693 is that it cannot be integrated into an embossed card. Another disadvantage is that the coupling of this device in read mode with another antenna does not meet the ideal conditions to obtain optimum coupling with a transponder.
Documents EP-A-1,031,939 and FR-A-2,777,141 describe an antenna circuit device for transponder mode operation having two electrically independent antenna circuits. In the device described in documents EP-A-1,031,939 and FR-A-2,777,141, a first antenna circuit consists of a conventional inductance and the transponder chip. A second antenna circuit consists of a coil winding forming an inductance associated with a planar capacitance called a “resonator”. The objective of the two embodiments is to allow amplification of the electromagnetic signal received by the “resonator” arrangement for the first antenna circuit comprising the transponder.
This device according to EP-1,031,939 and FR-2,777,141 has the disadvantage of coupling that is much too strong, without guaranteeing the efficiency of increased read distance. Worse still, when coupling efficiency is extremely strong, RFID communication between the reader and the transponder does not take place.
Additionally, the same remarks as for document U.S. Pat. No. 7,212,124 can be made. With a conventional “resonator” circuit, coupled by mutual inductance with a first antenna circuit comprising the transponder, the relationship is quasi-linear, to make things simple, between, firstly, the efficiency of reading distance or efficiency of electromagnetic field capture and, secondly, the surface of the 2 antenna circuits, their proximity, and their frequency tuning.
The advantage of the embodiments described in documents EP-A-1,031,939 and FR-A-2,777,141 is that maximum efficiency is obtained between the 2 antenna circuits, hence the greatest possible quality coefficient. We therefore arrive at the same remarks as for document U.S. Pat. No. 7,212,124.
Document EP-A-1,970,840 describes a device comparable with the two preceding devices described in documents EP-A-1,031,939 and FR-A-2,777,141, in that 2 resonators are used to amplify the received electromagnetic field. The same remarks therefore apply as previously made. In addition, the constraints indicated for documents EP-A-1,031,939 and FR-A-2,777,141 are all the higher and more difficult to overcome since the two resonators lie close to one another.
On the other hand, document U.S. Pat. No. 3,823,403 describes a tri-dimensional loop antenna which is used especially for VHF (from 30 MHz to 300 MHz), which is formed by a length of conductor, ideally by tubes, which is coiled into two or more turns and which is mounted and linked in its intrinsic design and operating for an aircraft by external currents carried by its support and or its structure over a conducting ground plane or metallic structure or in a cavity which may be air filled or loaded with a ferrite or a dielectric on an aircraft.
The length of this tri-dimensional VHF antenna for aircraft is close to the wavelength or the quarter wavelength like the standard antennas for VHF frequencies, in order to come the closest as possible to the desired resonance frequency. This tri-dimensional VHF antenna for aircraft is dedicated to high powers and allows to improve the electromagnetic radiation pattern compared to standard loop antennas or stub antennas or dipole antennas, especially by rising the length of the antenna.
This tri-dimensional VHF antenna for aircraft has no mechanical constraints about a planar design or very small volume to conform to integration in environments of often very small width. This tri-dimensional VHF antenna for aircraft has no electric and radiofrequency constraints about coupling, mutual inductance, decreasing of the near magnetic field, filtering of modulated data, self-feeding or feeding by external fields, and of load modulating, which are the own criterias and constraints of small RFID/NFC antennas at for example 13.56 MHz.
To increase the transmission of emitted or received energy by the antenna, it is possible to add an amplifier in the radio transmission or receiving chain, but this adds to the financial cost and available energy and entails probable distortion on the modulated HF signal.
It is also possible to increase the level of the signal emitted by the silicon, but this is often limited by integration, technological choices, and size.
It is also possible to reduce the internal consumption of the silicon, but current needs for signal cryptography safety, ever increasing memory capacity, and speed of task execution mean that the trend is more in the direction of increased energy consumption.
To increase the emitted or captured magnetic field, coupling, mutual inductance, it is also possible to increase considerably the number of antenna turns. This would increase the inductance of the antenna, the number of turns facing the antenna to be coupled, and hence mutual inductance and coupling. With very close distances between the 2 antennas (<2 cm) this is not an ideal solution either, since the mutual inductance would be very high, and would lead to ill-functioning of the RFID systems by introducing a very high quality coefficient Q and hence a very low bandwidth. For long distance operation (>15 cm) it would be an almost ideal solution, but the modulated HF signal would be filtered for RFID/NFC systems.
Finally, it is possible to act on the size of the antenna, but this is a variable which is rarely debatable and is often a constraint.